Hypogravity (low G) has been known to elicit many changes in automatic and endocrine functions which may be regulated by the hypothalamus. We investigated hypothalamic neuron activity during low-G simulation and its involvement on regulation of muscle electrolytes and Ca metabolism. 1) In unanesthetized rats, about 50% of neurons of the lateral hypothalamic area (LHA) showed changes in activity during horizontal suspension. About 40% of neurons of the paraventricular necleus (PVN) showed changes in activity during head-down tilt suspension. Inputs from barorecestors and/or vestibular system seem to affect both LHA and PVN neuron activities, while proprioceptive information mainly affect LHA activity. 2) Electrophysiological experiments revealed that polysynaptic excitation from the vestibular nucleus and monosynaptic inhibition from the fastigial nucleus were main inputs to the LHA. 3) Rats under conditions of long-term hypokinesia, potassium-deficient diet or DOCA (deoxy corticosteron acetate) -salt hypertension show increase and decrease in intracellular concentration of sodium and potassium ions, respectively, in muscles. Lesion of the ventromedial hypothalamus (VMH) or PVNreduced the electrolyte changes in the muscle. VMH activity through sympathetic nerve are involved in regulation of Na-K pump activity in the slow muscles, while endogenous Na-K ATPase inhibitor in the PVN and the anteroventral region of the III ventricle is involved in regulation of Na-K pump activity in the fast muscles. 4) LHA neurons have calcitonine sensitivity and central injection of calcitonin reduced plasma concentration of Ca. These findings suggest that the hypothalamus is involved in regulation of calcium as well as other electrolytes.